The present invention generally relates to semiconductor devices and more particularly a quantum semiconductor device including therein a quantum dot structure.
In a so-called bulk crystal where there is no confinement of carriers, it is well known that the density of state of the carriers increases continuously and parabolically with energy. In a quantum well structure in which carriers are confined one-dimensionally in a crystal, there appear discrete quantum levels as is well known in the art. In such a case, the density of state of the carriers changes stepwise. Because of such a restriction imposed on the distribution of the carriers, a quantum well structure provides a narrow spectrum when used for an optical semiconductor device such as a laser diode, and the efficiency of laser oscillation is improved. Further, a quantum well structure is used in electron devices having a resonant tunneling barrier such as an RHET (Resonant Hot Electron Transistor) as an energy filter of carriers.
In a quantum well wire structure in which the degree of confinement of the carriers is increased further, the density of state of the carriers in the crystal is modified such that the density of state is a maximum at the bottom edge of each step. Thereby, the sharpness of the spectrum is increased further.
In an ultimate quantum dot structure in which the degree of carrier confinement is increased further, the density of state becomes discrete in correspondence to the discrete quantum levels. A system having such a discrete energy spectrum, in which transition of carriers occurs only discontinuously or stepwise, provides a very sharp spectrum when used for an optical semiconductor device even in a room temperature environment where the carriers experience substantial thermal excitation.
Further, the quantum dot structure is drawing the attention of scientists in relation to the problems of fundamental physics such as a phonon bottleneck problem of energy relaxation.
Conventionally, a quantum well structure has been formed readily and with reliability by using an MBE (Molecular Beam Epitaxy) process or an MOCVD (Metal Organic Chemical Vapor Deposition) process such that a very thin quantum well layer is sandwiched between a pair of barrier layers. On the other hand, a quantum well wire has been formed by growing thin semiconductor layers laterally on a so-called inclined semiconductor substrate having a stepped surface structure. Alternately, a quantum well wire may be formed by applying an electron beam lithography to an ordinary, one-dimensional quantum well structure.
Thus, various attempts have been made to form quantum dots by using an inclined substrate similarly to the case of forming a quantum well wire. However, such conventional attempts have faced a problem of controlling the stepped surface of the inclined substrate. Further, there tends to occur a mixing of elements at the boundary of the quantum dots formed in such a manner. Thereby, a desired sharp transition of the composition is difficult in the quantum dots formed as such. Thus, there are few successful attempts in the approach that use an inclined substrate in combination with lateral epitaxial growth process of semiconductor layers. In addition, an approach to use electron-beam lithography to form a quantum dot is also unsuccessful due to the damage caused in the epitaxial layers forming the quantum dot or a barrier layer at the time of patterning.
On the other hand, there has been a discovery that a quantum dot can be formed easily by using a so-called S-K (Stranski-Krastanow) mode growth that occurs in a strained heteroepitaxial system such as an InAs/GaAs heteroepitaxial structure at the initial period of the heteroepitaxial growth. In an S-K mode growth, quantum dots are formed in the form of discrete islands on a substrate. For example, it is reported that an MBE growth of an InGaAs layer having an In-content of 0.5 on a GaAs substrate with a thickness of several molecular layers, results in a formation of islands of InGaAs each having a diameter of 30-40 nm on the GaAs substrate (Leonard, D., et al., Appl. Phys. Lett. 63, pp.3203-3205, 1993). Further, it is reported that islands of InGaAs having a diameter of 15-20 nm are formed on a GaAs substrate by an ALE (Atomic Layer Epitaxy) process with a mutual distance of about 100 nm (Mukai, K., et al., Jpn. J. Appl. Phys., 33, pp.L1710-L1712, 1994). Further, a similar quantum dot can be formed also by a MOVPE process (Oshinowo, J. et al., Appl. Phys. Lett. 65,(11), pp.1421-1423, 1994).
As the formation of a quantum dot in such a strained heteroepitaxial system is controlled by a strain energy formed at the heteroepitaxial interface, the formation of the quantum dot is substantially simplified as compared with the conventional process discussed previously. Further, the formation of a quantum dot on a strained heteroepitaxial system does not require a patterning process and is inherently free from damages. There is already a report claiming successful observation of a photoluminescence (PL) (Leonard, D., et al., op. cit.), in which it is reported that a broad PL peak is confirmed in the vicinity of 1.2 eV with a substantial intensity.
In the quantum dots formed by the S-K mode growth, however, the observed PL peak, although having a substantial intensity, spreads or diffuses substantially. For example, the half-height width FWHM (Full Width at Half Maximum) of the PL peak spreads over a range of 80-100 meV, probably due to the poor control of the size of the individual quantum dots. It should be noted that the PL wavelength corresponding to the foregoing PL peak energy is about 1.1 .mu.m, which is shifted substantially on a shorter wavelength side with respect to the wavelength of 1.3 pm that is used commonly in the field of optical telecommunication and optical information processing. With the conventional S-K mode quantum dots, it has been difficult to tune the PL wavelength as necessary. As will be described later, the size of the quantum dot itself can be controlled to some degree by controlling the deposition temperature. However, the size of the quantum dots formed in such a manner changes variously. It is believed that it is such a variation of the size of the quantum dots that causes the foregoing unwanted spreading of the PL peak. Further, it is believed that conventional quantum dot structure thus formed by the S-K mode growth includes a substantial number of quantum dots that do not contribute to the photon emission.
Recently, Farad et al., (Farad. S., Appl. Phys. Lett., 68(7), pp.991-993, Feb. 12, 1996) has reported a successful observation of a PL wavelength in the 1.5 .mu.m-band for an S-K mode quantum dot of InAs formed on an AlInAs buffer layer, which in turn is provided on an InP substrate with a lattice matching therewith. In this case, however, the value of FWHM for the observed PL spectrum exceeds 110 meV, indicating that there still remains a substantial problem in the size control of the individual quantum dots.
Thus, conventional S-K mode quantum dots have failed to provide a sharp spectrum of photon emission in the wavelength band of 1.3 .mu.m or 1.5 .mu.m, which is important for industrial applications. Further, a similar problem occurs also in electron devices such as an RHET. An energy filter formed by conventional S-K mode quantum dots performs poorly when the S-K mode quantum dots are used in a resonant-tunneling barrier of an RHET. In such a case, the desired sharp tunneling effect is not obtained.
Meanwhile, the inventor of the present invention discovered a phenomenon, in a S-K mode growth of quantum dots, in that quantum dots grown on an intermediate layer covering quantum dots of a lower layer, align with the corresponding quantum dots of the lower layer (Sugiyama, Y., et al., Jpn. J. Appl. Phys. 35, Part I, No.2B, pp.365-369, February, 1996). Based on the discovery, the inventor of the present invention has proposed, in the U.S. patent application Ser. No. 08/753,598, which is incorporated herein as reference, a quantum semiconductor device comprising, a semiconductor substrate, an active layer formed on the semiconductor substrate and including a quantum structure, the quantum structure comprising a plurality of intermediate layers stacked on each other repeatedly, each of the plurality of intermediate layers being formed of a first semiconductor crystal having a first lattice constant; each of the intermediate layers including a plurality of quantum dots of a second semiconductor crystal having a second lattice constant different from the first lattice constant, the second semiconductor crystal forming thereby a strained system with respect to the first semiconductor crystal, each of the quantum dots in an intermediate layer having a height substantially identical with a thickness of the intermediate layer; a quantum dot in an intermediate layer aligning with another quantum dot in an adjacent intermediate layer in a direction perpendicular to a principal surface of the semiconductor substrate; each of the plurality of intermediate layers having a thickness equal to or smaller than a Bohr-radius of carriers in the intermediate layer.
It is believed that the foregoing alignment of the quantum dots is caused as a result of accumulation of the strain in the intermediate layer in correspondence to the quantum dots covered by the intermediate layer. More specifically, the atoms constituting the semiconductor quantum dots are deposited preferentially on the part of the intermediate layer where the accumulation of the strain is caused. The quantum dots thus aligned perpendicularly to the substrate develop a quantum mechanical coupling with each other to form an effectively single quantum dot having an effectively uniform size.
FIG. 1 shows a quantum structure 3 that includes the vertically aligned S-K mode quantum dots schematically.
Referring to FIG. 1, the quantum structure 3 is formed on a buffer layer 2 of GaAs that in turn is formed on a (100)-oriented surface of a GaAs substrate 1. The buffer layer 2 is formed with a thickness of 400 nm, and a plurality of GaAs intermediate layers 3a are stacked repeatedly on the foregoing buffer layer 2. Each of the intermediate layers 3a carries therein a plurality of quantum dots (islands) 3b of InAs, wherein each of the quantum dots 3b are isolated from other quantum dots 3b in each of the intermediate layers 3a.
It should be noted that InAs has a lattice constant different from that of GaAs forming the buffer layer 2 by about 7%. In other words, the quantum dots 3b form a strained heteroepitaxial system with respect to the buffer layer 2 and hence the substrate 1. In such a strained heteroepitaxial system, there appears a S-K mode growth at the initial period of epitaxial growth when forming an InAs layer, wherein such an S-K mode growth leads to the formation of the island structure of InAs on the surface of the GaAs buffer layer 2.
In the illustrated structure, it should be noted that the GaAs intermediate layer 3a buries the islands 3b of InAs, and the deposition of the intermediate layer 3a and the island 3b is repeated. Each of the islands 3a typically has a diameter of about 20 nm and a height of about 5 nm and forms a quantum dot that confines carriers therein three-dimensionally in combination with the intermediate layer 3a having a larger bandgap and thus acting as a barrier layer.
The inventor of the present invention has discovered previously that the quantum dots 3b align generally perpendicularly to the principal surface of the substrate 1 when the intermediate layer 3a and the quantum dots 3b are deposited repeatedly and alternately as indicated in FIG. 1 (Sugiyama, Y., et al., op. cit.). As explained already, this phenomenon of vertical alignment of the quantum dots 3b is explained by the accumulation of strain in the part of the intermediate layer 3a that covers the underlying quantum dots 3b. The strain is induced by the difference in the lattice constant between the intermediate layer 3a and the quantum dot 3b, wherein the strain thus induced in turn induces an island growth of InAs on the intermediate layer 3a in correspondence to the part where the strain is accumulated.
FIGS. 2A and 2B show a photoluminescence (PL) spectrum obtained for the quantum structure of FIG. 1 at 77 K, wherein the PL spectrum of FIG. 2A is obtained for the quantum structure in which there is only one layer of the quantum dots 3b of InAs, while FIG. 2B shows the PL spectrum of the quantum structure also at 77 K in which the quantum dots 3b of InAs are stacked to form five layers.
Referring to FIG. 2A, it can be seen that the PL spectrum of the quantum structure is low and diffused, indicating that there is a substantial variation in the size of the individual quantum dots 3b. Further, it is noted that the central PL wavelength is shorter than 1.1 .mu.m, indicating that the quantum structure cannot be used for optical telecommunication or optical information processing that uses a wavelength of 1.3 .mu.m or 1.5 .mu.m.
On the other hand, the structure in which the quantum dots 3b are stacked in five layers provides a very sharp PL spectrum as indicated in FIG. 2B. Further, FIG. 2B clearly indicates that the central wavelength is shifted in the shorter wavelength side as compared with the spectrum of FIG. 2A. The result of FIG. 2B supports the interpretation noted before that the stacked quantum dots develop a quantum mechanical coupling to form an effectively single, large quantum dot.
On the other hand, the result of FIG. 2B indicates that the maximum wavelength that can be reached according to such a stacking of the quantum dots is limited to about 1.2 .mu.m. A further increase of the PL wavelength requires a further increase in the number of stacked layers of the quantum dots, while such an arbitrary increase of the stacking of the quantum dots is difficult to achieve.